1. Field of the Invention
The invention relates to a method and an apparatus for determining an aerosol particle size distribution.
2. Description of the Related Art
Particles in the atmosphere or in other gases are known as aerosols or aerosol particles. They may be formed in the atmosphere by chemical reactions of natural and anthropogenic gaseous precursors or they may be formed by combustion in engines and power plants or may origin from emissions from facilities like paint shops that use or create aerosol particles. These aerosol particles play a significant role in both air pollution (smog) and climate balance (cloud formation, absorption, emission and scattering of radiation). Furthermore, some forms of aerosol particles like for example fine particles can be health threats. It is therefore of great interest to provide methods and devices for measuring the sizes and compositions of aerosol particles efficiently and with high sensitivity.
Methods and apparatuses pertaining to the above mentioned technical field are known. For example, U.S. Pat. No. 6,040,574 (Jayne, Worsnop, Kolb) describes an apparatus for determining the size of aerosol particles coupled to a mass spectrometer and a corresponding measuring method. In this apparatus, an aerosol particle beam is formed by passing gas with entrained aerosol particles through a unit such as for example the one described in U.S. Pat. No. 5,270,542 (McMurry et al.). Subsequently, pulses are formed of the aerosol particle beam and the time of flight of the aerosol particles is measured in order to determine the size of the aerosol particles.
The disadvantage of such known apparatuses and methods is that in order to measure the time of flight of the aerosol particles, one pulse of aerosol particles is generated and the time of flight of the aerosol particles is measured. Subsequently, the next pulse of aerosol particles is generated and the time of flight of the aerosol particles in this next pulse is measured. As a consequence of forming such a pulsed aerosol particle beam from a continuous aerosol particle beam, the resolution and sensitivity of the measurement cannot both be optimized at the same time. It is only possible to form narrow pulses of the aerosol particle beam in order to provide a higher flight time resolution by paying with a reduced duty cycle and consequently a reduced sensitivity. Or it is only possible to form broad pulses of the aerosol particle beam in order to provide a shorter duty cycle and enhanced sensitivity by paying with a reduced flight time resolution due to the broad pulses.